This invention relates to a digital wave observation apparatus and, more particularly, to an apparatus for always displaying the currently set trigger level.
A typical example of a conventional digital wave observation apparatus is the digital oscilloscope shown in FIG. 1. Referring to FIG. 1 reference numeral 1 denotes a trigger signal generator. In the trigger signal generator 1, a trigger level set by a volume control 2 is compared by an analog comparator 3 with an input signal. When the input signal is larger than the trigger level set by the control 2, a trigger signal is generated by the generator 1. The trigger signal is supplied to a signal fetching controller 4 which then supplies a control signal to a wave memory 6 to cause the memory 6 to write the signal. Digital wave data obtained by an A/D converter 5 are sequentially stored in the memory 6. The digital wave data is read out from the memory 6 under the control of a display controller 7. The readout wave data is displayed on a display 8.
In the conventional oscilloscope, an input signal is used as a trigger source. A trigger point is determined by a preset delay time, and the trigger level can be determined by reading out the input signal value at the trigger point from the wave memory 6. The trigger level can be displayed as a horizontal cursor like an abscissa line or a marker on a screen of the display 8.
In the conventional digital oscilloscope, generation of the trigger signal from the generator 1 has a sequential relationship with fetching of the input signal wave data in the memory 6. When the trigger level is properly set, the trigger level can be displayed on the screen of the display 8, as described above. However, when the trigger level is not properly set, the trigger level cannot be properly displayed on the screen of the display 8.
FIG. 2 shows a display example wherein a delay is effected in the negative direction and a proper trigger level is set. Point A in a waveform 10 displayed on a display screen 9 represents the trigger point. A negative delay time prior to triggering is given by a time interval .DELTA.T between point B and point A. Wave data of the input signal between point B and point C is stored in the memory 6. Since the negative delay time .DELTA.T is predetermined, the trigger point A is determined by the delay time .DELTA.T, and the current trigger level can be detected. Therefore, this trigger level 12 is displayed as the solid horizontal line of FIG. 2.
However, when the trigger level 12 is not properly set as shown in FIG. 3, or when a positive delay time is set as shown in FIG. 4, an input signal corresponding to the trigger point cannot be stored in the memory 6. Therefore, neither the trigger level corresponding to the trigger point is present nor the trigger level itself can be displayed on the screen 9 of the display 8.